$highlight-color: #f90;
$basic-border: 1px solid black;
#app {
  background-color:  $highlight-color;
  border: $basic-border;
}     
nav {
  width: 100px;
  color: #F90;
  background-color: black;
}
#content {
  article {
    h1 { color: #1dc08a }
    p {  font-style: italic; }
    a {
      color: blue;
      &:hover { color: red } //# 相当于a:hover{}
    }
  }
  aside { background-color: #f90 }
}
